The only themes we've seen so far have been the Dark Sun ones, but they already allow a limited amount of multiclassing - and at least in some ways it's better than standard multis. You aren't required to take a theme that matches the role or power source of your class. And since you can swap in the theme's powers freely (ie: without taking up feat slots) you can have a much more hybridized character. Add in the theme-based feats and paragon paths and I think you've got a pretty close analog of multiclassing. The only real issue is that each theme has only one set of powers, although they do scale pretty well, at least to mid-paragon, as far as I've seen.